General background of nuclear magnetic resonance well logging [also called magnetic resonance (MR) logging] is set forth in copending U.S. patent application Ser. No. 08/873,582, assigned to the assignee hereof, and in U.S. Pat. No. 5,023,551. Briefly, in nuclear magnetic resonance operation, the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by .omega..sub.0 =.gamma.B.sub.0, where B.sub.0 is the strength of the static field and .gamma. is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, the spin lattice relaxation time. For hydrogen nuclei, .gamma./2.pi.=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation, T2, called the spin-spin relaxation time. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess near the Larmor frequency. However, because of molecular interactions, each nuclear spin precesses at a slightly different rate. T2 is a time constant of this "dephasing".
A widely used technique for acquiring NMR data both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. [See Meiboom, S., Gill, D., 1958, "Modified Spin Echo Method for Measuring Nuclear Relaxation Times", Review of Scientific Instruments, 29, 688-691]. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then a one hundred eighty degree pulse is applied to keep the spins in the measurement plane, but to cause the spins which are dephasing in the transverse plane to refocus. By repeatedly reversing the spins using one hundred eighty degree pulses, a series of "spin echoes" appear, and the train of echoes is measured and processed, for example to obtain a T.sub.2 distribution of fluid components of the formations.
Magnetic resonance logging has added a new dimension to formation evaluation. The MR measurement is sensitive to total fluid content, to the intrinsic properties of the fluids, and to the environments in which the fluids reside in the pore space of porous rock. There are many applications of magnetic resonance tools, and among the most common and important are the determination of capillary and clay bound water volumes and the estimation of permeability. Bound water analysis is of central interest to the log interpreter because it helps predict the potential water cut of a formation volume, and is a major factor in the perforation decision. A continuous permeability log can provide an estimation of potential production rate and also indicates flow barriers and thief zones. These log outputs are difficult to obtain reliably with other logging tools, and hence may be termed "MR-unique".
One limitation of MR tools is their slow logging speed. Magnetic resonance logging uses a cyclic measurement consisting of a wait time followed by an echo acquisition period. Conventional MR data are presently acquired with wait times sufficiently long to substantially polarize all fluid protons in the formation. Protons in gas, light oil, oil base mud filtrates, and vug water polarize very slowly. Therefore, very long wait times have been used when logging formations in which those fluids are present. The long wait times necessarily contribute to slow logging speeds. Thus, while MR tools provide important additional information, it can be at the cost of wellsite efficiency.
Precision of MR logging outputs (e.g. porosity) can be enhanced by increasing the stacking of the data. In conventional practice, this requires either degrading the vertical resolution, or logging more slowly to collect more data within a given depth interval.
Heretofore it has been considered difficult or impossible to estimate residual oil saturation in wells drilled with oil base mud. This is particularly true when the native oil has a low downhole viscosity, which renders it difficult or impossible to distinguish native fluids from invaded fluids on the basis of existing magnetic resonance T.sub.2 measurements.
It is among the objects of the present invention to overcome limitations of prior art MR techniques by determining residual oil saturation and other formation characteristics in wells drilled with oil base mud. It is also among the objects of the invention to improve logging speed (e.g. for a given precision and/or vertical resolution) of MR logging. It is also among the objects of the invention to provide a technique for determining when formation permeability may be overestimated in certain types of formations.